The present invention relates generally to methods and apparatus for use in soil remediation, and more particularly to methods and apparatus for determining the depth of a gamma emitting element beneath the surface of a volume of material.
With the growing awareness of contamination of large tracts of land with chemically or radioactively hazardous elements, there is a corresponding international effort to initiate remediation activities to restore affected regions to an environmental status considered acceptable. To this end, soil washing and other methods are being developed. For these methods to be technically efficient and cost effective, it is necessary to accurately identify where the contamination in a field is located.
U.S. patent application Ser. No. 958,215, filed Oct. 8, 1992, titled "Prompt Gamma Neutron Activation Analysis System," which is hereby incorporated by reference, discloses the use of Prompt Gamma Neutron Activation Analysis (PGNAA) in soil remediation. Experimental and analytical data presented therein demonstrate that the disclosed methods and system are capable of measurements of trace element concentrations within a material sample by achieving extremely high signal-to-noise ratios.
PGNAA employs neutron-induced reactions. Such reactions can be divided into two broad categories--threshold reactions and exoergic reactions. For threshold reactions, energy in the form of neutron kinetic energy is required to supply a certain minimum energy to the reaction system before the reaction can proceed. Neutrons with energies below this minimum threshold energy are incapable of inducing the nuclear reaction. For exoergic reactions, the threshold is zero; that is, neutrons with all energies are capable of inducing the reaction.
FIG. 1 illustrates the process of neutron activation at a nuclear level. A neutron of energy E from a source 16 collides with the nucleus of an atom in the sample and initiates a reaction. For a neutron of thermal energy (0.0252 eV), the reaction might be absorption of the neutron into the nucleus, creating the next higher mass isotope of that element. If the neutron is more energetic (e.g., with several mega-electronvolts of kinetic energy), other nuclear reactions are possible. These other reactions include reactions wherein nuclear transmutation to another element occurs. In each of these cases, the residual nucleus is left in a highly excited internal state, and decays to its ground state almost instantaneously, emitting a gamma ray of several mega-electronvolts of energy. The energy of this gamma ray is uniquely characteristic of the quantum structure of the residual nucleus, and thus is a signature of the original target nucleus. The number of atoms of each of the elements of interest in a sample can be estimated by detecting (with a detector 12) and collecting the spectrum of gamma rays emitted by the sample and integrating the appropriate peaks.
Three methods for inferring the depth distribution of a contaminant element in a field of soil or other matrix material are disclosed by the above-cited U.S. patent application Ser. No. 958,215. These methods are summarized below:
1. The first method uses the inherent sensitivity bias of fast neutron-induced nuclear reactions for interrogating shallow depths (several inches) versus the deeper interrogation which is characteristic of thermal neutron-induced nuclear reactions.
2. The second method is based on the fact that, if a neutron-induced reaction on a particular nuclide of a contaminant element gives rise to more than one gamma ray, each with a known branching ratio, and if the energies of two or more such gamma rays are significantly different, the different amounts by which the gamma rays attenuate in propagating through the soil back to a detector will alter the ratio of their detected signals by an amount which is a unique signature of the average depth of the contaminant aggregate.
3. The third method makes use of the successive placement of none, one, or more reflectors above the neutron tube. The reflectors are large masses of highly moderating (low atomic mass constituent) materials. Graphite and polyethylene are examples. Such materials have the effect of utilizing neutrons which otherwise would have travelled upward from the neutron tube, significantly lowering their energy by collisions with the nuclei of the atomic constituents and reflecting them back toward the ground. The result is that the thermal neutron flux can be tailored to be highest in the shallow (several inches) depths of the soil with the reflectors, versus peaking at 6 to 10 inches (15.2 to 25.4 cm) below the soil surface in the absence of a reflector. Thus, even using only thermal neutron-induced nuclear reactions, it becomes possible to infer a depth distribution of a contaminant using thermal neutron PGNAA.